The higher #s caps allow the temps to run higher without turning to steam (think of a pressure cooker)
The cap seal is two stage. lower seal at spring is the 16 or 18 psi setting and will unseat when the pressure hits that number and send the fluid out the overflow line and a coolant blood trail in front of the L/front tire. From your post you are seeing fluid at the cap itself. The upper seal only comes into play once the lower opens up.
So you really have two issues:
The lower seal is either leaking or you have a Overflow tank problem. Take a look at the seating area, Or you are really over heating.
The Upper seal should not be leaking regardless of what cap #s you choose and if you are overheating or not. Same deal take a close look at the overflow tank flanges.
That's a brass tank and there could be some crud on the lower seal seating area. Cleaning it up with blue Scotchbrite and a bit of Naval Jelly (15% phosphoric acid) will help the sealing.
Get down to fender level and eyeball the tank top neck flange for bent areas. If you have a large enough socket that will mate up to the top sealing area. Put it on top the neck flange shine a flashlight down the center look that he periphery of the socket wall / flange for light leaks at the neck indicating a low spot.
I take it your fans are functioning ok, then even in manual fans ON you've never had overheating issues?
Are you still seeing the same water temp you're used to on the gauge?
Another telltale of overheating: The the
oil press will dip slightly as the viscosity lowers. (Not always especially if you have an
oil cooler.)
But if you have an
oil temp gauge it will be higher than normal.